1. Field of the Invention
The present invention relates to a piezoelectric element, more particularly to a piezoelectric element used in technological fields such as data processing and communications and functioning as a resonator, a filter and the like.
2. Description of the Related Art
A piezoelectric element includes, for instance, a piezoelectric member made of piezoelectric ceramic. This type of piezoelectric member is adapted to vibrate in a square type vibration mode or a thickness vibration mode and is widely used.
FIG. 10 shows a piezoelectric element 1 adapted to vibrate in a square type vibration mode. The piezoelectric element 1 includes a square plate-like piezoelectric member 2, and electrodes 3 and 4 are provided on the major surfaces of the piezoelectric member 2.
The piezoelectric member 2 is polarized in the direction indicated by an arrow 5, and square type vibrations are excited by applying an electrical field between the electrodes 3 and 4 via terminals 6 and 7.
FIG. 11 shows a piezoelectric element 8 utilizing an unstiffened vibration mode. The piezoelectric element 8 includes a thin plate-like piezoelectric member 9, and electrodes 10 and 11 are provided on the major surfaces of the piezoelectric member 9.
The piezoelectric member 9 is polarized in the direction indicated by an arrow 12, and longitudinal vibration in the unstiffened mode is excited by applying an electrical field between the electrodes 10 and 11 via terminals 13 and 14.
FIG. 12 shows a piezoelectric element 15 utilizing a stiffened vibration mode. The piezoelectric element 15 includes a thin plate-like or rod-like piezoelectric member 16, and electrodes 17 and 18 are provided on the major surfaces of the piezoelectric member 16.
The piezoelectric member 16 is polarized in the direction indicated by the arrow 19, and longitudinal vibration in a stiffened mode is excited by applying an electrical field between the electrodes 17 and 18 via terminals 20 and 21.
However, the piezoelectric elements 1, 8 and 15 shown in FIG. 10, FIG. 11 and FIG. 12 respectively have the following disadvantages.
Firstly, the piezoelectric element 1 shown in FIG. 10 is comparatively large, which prevents a circuit or device incorporating such element 1 from being made small. For instance, when the piezoelectric element 1 is used as an AM middle frequency filter in a 455 kHz radio, the piezoelectric member 2 must be a square plate-like piezoelectric member having approximate dimensions of 5 mm.times.5 mm, which is a considerable barrier to miniaturizing an electronic circuit or device incorporating such piezoelectric member 2.
Furthermore, in the case of the piezoelectric element 8 shown in FIG. 11, when the piezoelectric member 9 has a thin plate-like shape which is approximately 4 mm in length in order to make the piezoelectric element 8 small, a wide filter pass band cannot be achieved.
Furthermore, in the piezoelectric element 15 shown in FIG. 12, when the piezoelectric member 16 has thin rod-like shape of, for instance, approximately 4 mm in length in order to make the piezoelectric element 15 small, impedance is too high and the impedance cannot easily be matched with the impedance of a circuit incorporating the element 15.